Regulation of the Gut-Brain Axis by Guanylyl Cyclase C

The gut-brain axis is a multi-modal bi-directional communication system integrating gastrointestinal signals into the central nervous system. The gastrointestinal tract relays information about nutritive signals, motility, microbial byproducts, and infection through a host of modalities including hormonal signaling and direct synaptic integration with peripheral neurons. Obesity, visceral pain syndromes, mood disorders, and neurodegenerative diseases have all have links to dysregulation of gut-brain signaling. Identifying molecular targets to repair gut-brain dysbiosis is essential to finding safe and effective treatments for these debilitating diseases. One such target is guanylyl cyclase C (GUCY2C), a transmembrane receptor found on the luminal aspect of the intestinal epithelium, first described for its role in intestinal secretion. GUCY2C is the receptor for two cognate hormones, guanylin (GUCA2A) and uroguanylin (GUCA2B), secreted by the colon and small intestine, respectively. Intriguingly, loss of GUCY2C ligands is associated with both obesity and irritable bowel syndrome, and mouse models deficient in GUCY2C have been shown to be hyperphagic and obese. Reconstituting GUCY2C signaling by oral administration of ligand relieves symptoms of irritable bowel syndrome but has no effect on satiety. Conversely, intravenous infusion of GUCY2C ligand induces satiety, decreasing food intake. Here, we attempt to untangle the multi-modal role of GUCY2C in the gut-brain axis by describing unique anatomical location of GUCY2C in brain and rare intestinal cells. We map GUCY2C protein and mRNA in the brain, demonstrating that GUCY2C is transcribed and translated in the ventral premammillary nucleus (PMV) of the hypothalamus, as well as in dopaminergic nuclei in the midbrain. From these sites, GUCY2C protein is carried to distinct nuclei through axonal projections. Importantly, we find that GUCY2C in the ventral premammillary nucleus is expressed primarily in neurons expressing leptin receptor (LepR), and projects to identical nuclei as LepR+ neurons. These neurons are implicated in feeding regulation and provide one branch of the GUCY2C gut-brain axis where hormonal GUCY2C ligand released post-prandially interacts with hypothalamic GUCY2C to induce satiety. We also find that GUCY2C is most highly expressed in a rare enteroendocrine intestinal cell type called neuropod cells. These cells directly interact with peripheral neurons and are highly reactive to GUCY2C ligand. We find that these cells form another branch of the GUCY2C gut-brain axis, by transducing luminal signals from GUCY2C ligands to dorsal root ganglia neurons, decreasing visceral pain signals to the spinal cord. This affect is unique to neuropod cells, as eliminating GUCY2C only in neuropod cells eliminates analgesic effects of oral GUCY2C ligands. Together, these two branches of the GUCY2C gut-brain axis provide evidence for GUCY2C activity outside of intestinal enterocytes regulating multiple homeostatic signaling mechanisms. The expression of GUCY2C in these newly discovered anatomic locales also provides attractive opportunities for combinatorial therapeutics to enhance the anorexic or analgesic effects of GUCY2C ligands while minimizing the secretory effects of these drugs in intestinal enterocytes